How could you justify having the federal government regulate something that is clearly a state issue?

I know this seems like a weird question, but it’s for an academic assignment. An example would be if the federal government abolished smoking bans. Smoking is a blatant state decision, but there has got to be a legitimate reason you could argue that gives Washington the rights to do so. (Other than “because we freaking feel like it”) :D Any help would be much appreciated…